Grav plugin form submit (Mailchimp)

I'm trying to create a plugin to add email submitted via the newsletter form to the Mailchimp list. My question is on what event should it be initialised? I know there's this list of events available, but it's not clear to me at what point it should be initiated?

If you don't want it to interact with any other actions, use onPluginsInitialized

Thank for your advice. The plugin is starting to do what it supposed to. However, currently I have a different problem: I'm trying to tweak AJAX call, so it prevents page refresh and handle messages.
The code below was working before I've created calls to MailChimp API.

Currently I'm receiving this warning:

Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help http://xhr.spec.whatwg.org/

Any idea what's wrong and how to fix it?

$(document).ready(function() {
const newsletterForm = $('.js-form-newsletter');
newsletterForm.submit(function(e) {
    // prevent form submission
    e.preventDefault();

    // submit the form via Ajax
    $.ajax({
        url: newsletterForm.attr('action') + '?submit_newsletter=1',
        type: newsletterForm.attr('method'),
        dataType: 'html',
        data: newsletterForm.serialize(),
        success: function(result) {
            $('#newsletter-form-result').html(result);
            if ($('#newsletter-form-result div.alert.notices').hasClass('green')) {
                $('.newsletter_form--hide-js').hide();
                $('.newsletter__info-text').hide();
            }
        }
    });
});
});

That is because you most probably want to make a cross origin ajax call (that means: url / domain of the call is a domain different from you.

Look at https://api.jquery.com/jQuery.ajax/ in the section "async".

Would love to see this in action. I wrote a custom plugin back in laravel and it worked swimmingly but haven't attempted in grav yet :)

I have wrote a plugin that is functional, but not yet ready for release to the public. I still need to write the twig, css, js to accompany this.

https://github.com/apotropaic/grav-plugin-email-subscribe

Hoping to get it done soon! I'm writing it to support multiple services, not just mailchimp.